This application relates to optical gratings, and more particularly, to sampled optical fiber Bragg gratings (FBGs) for optical wavelength-division multiplexed (WDM) devices and systems.
Optical gratings may be formed in optical conduits such as waveguides and fibers by making spatial periodic structures in the optical conduits along the direction of light propagation. Assuming such an optical conduit has an index of refraction, n, and the periodic structure has a spatial pattern with a spatial period xcex9, an optical wave may be reflected by interacting with this periodic structure when a Bragg phase-matching condition of xcex=2nxcex9 is satisfied, where xcex is the wavelength of a reflected optical wave. Optical waves that fail to satisfy the Bragg condition do not efficiently interact with the periodic structure to produce reflected signals. Thus, these optical waves essentially transmit through the grating.
Such a grating may have a constant grating parameter nxcex9 to produce only a single Bragg reflection at a wavelength of xcex=2nxcex9. Alternatively, the grating parameter nxcex9 may vary, i.e., xe2x80x9cchirpxe2x80x9d, with a position z along the optic axis of a waveguide or fiber so that different spectral components at different wavelengths may be reflected at different positions to experience different delays. This chirped grating can be used for a number of applications, including dispersion compensation in fiber systems.
A second periodic sampling structure may be further included in the grating to superimpose over the above underlying grating structure. This second sampling structure may have a sampling period greater than that of the underlying grating. One effect of this second sampling structure in the spatial domain is to produce a series of discrete reflection channels centered at different wavelengths in the frequency domain, e.g., at the specified wavelengths for WDM applications as defined by the Internal Telecommunications Union (ITU). The reflection channels essentially correspond to different discrete terms in the Fourier transform of the spatial sampling structure.
One embodiment of a sampled grating of this disclosure includes a wave-guiding conduit to transport optical energy along an optic axis, a grating structure formed in the wave-guiding conduit, and a sampling structure formed in the wave-guiding conduit to superimpose a phase sampling pattern over the grating structure. The grating structure is designed to vary spatially with a grating period along the optic axis to reflect an optical spectral component in the optical energy that satisfies a Bragg phase-matching condition.
The phase sampling pattern has a sampling period greater than the grating period. The sampling period may be a constant or may vary spatially along the direction of the grating. Each period of the phase sampling pattern includes a plurality of contiguous, discrete spatial phase segments along the optic axis. The grating structure changes a phase between at least two adjacent phase segments. Within each phase segment, the grating structure does not change phase.
Alternatively, each period of the phase sampling pattern may be designed to continuously change the phase of the grating structure.